Trends in microelectronic devices are toward increasing miniaturization, circuit density, operating speeds and switching rates. These trends have directly impacted the complexity associated with the design and manufacture of dies, microelectronic devices, which include the microelectronic die and a substrate, microelectronic packages, which include the microelectronic device, as well as computing devices in general. Examples of computing devices include, but are not limited to servers, personal computers and “special” purpose computing devices. Personal computers may have form factors, such as desktop, laptop, tablet, and the like. “Special” purpose computing devices may include set top boxes, personal digital assistants, wireless phones, and the like.
Accordingly, substrates, including but not limited to those used in microelectronic packages, have also evolved to enable the microelectronic devices to operate at higher speeds and efficiencies. Substrates include, but are not limited to, interposers, printed circuit boards, motherboards, and the like. One such advancement includes the use of conductive core substrates. One example of a conductive core substrate is a metal core substrate, which comprises a single or multiple metal layers encapsulated in a dielectric material. Metals used in metal core packages include, but are not limited to, copper, molybdenum, copper-Invar-copper and other conductive metals. Metal core substrates have become more prevalent due to their low coefficient of thermal expansion (CTE), low inductance, low resistance, high thermal conductivity and lower cost. The metal core also provides structural support to allow the substrate to carry large and heavy components, and to function in environments where shock, vibration, heat, and survivability are a factor.
Another advancement in substrate technology is the incorporation of differential signaling for the transmission of signals/data to and from a microelectronic die. Differential signaling provides a pair of conductive pathways, in one example, also known as traces, formed within and/or on the substrate to conduct the signal. A fist trace typically carries a positive signal, and a second trace carries a signal that is of equal magnitude, but opposite in phase, i.e. a negative signal. Differential signaling provides a number of benefits, including, but not limited to, lower voltage swings, faster switching rates, reduced power consumption, and reduced electromagnetic interference (EMI). Differential signals carried on the first and second traces are also less sensitive to electrical cross talk or interference, and have better overall noise immunity. For example, noise generated by spurious conditions within the microelectronic package or noise generated from an outside source adds to both signals equally. Thus, when the receiver subtracts the noise in the negative signal from the noise in the positive signal, the noise in each signal trace effectively cancels out. This is known in the art as “common mode noise assumption.”
With differential signaling, it is important to design the trace pairs in such a way that the characteristic impedance of the first and second trace is equal and constant. Substrate design configurations often include the first trace and the second trace to traverse from one substrate layer to another, which can require passing through the conductive core.
FIG. 5 is a top view of a portion of a current metal core substrate 10 comprising a first trace 12, adapted to carry a positive signal, that runs generally parallel to a second trace 12′, adapted to carry a signal having an equal but opposite magnitude. First and second traces 12, 12′ traverse a layer of the substrate. The first trace 12 interconnects with a first via 14 that extends through the metal core 10. Second trace 12′ interconnects with a second via 14′ also extending through the metal core 10. First and second vias 14, 14′ allow the differential signals to pass through metal core 10 to a different layer.
FIG. 6 is a side view of the metal core 10 in accordance with FIG. 4, showing the path of the first trace 12 from a first side of metal core 10 to a second side of metal core 10. Trace 12 traverses a layer 15 above the metal core 10, passes through metal core 10 using the path formed by a via 14 extending through the metal core 10, such as a plated through hole (PTH), and then again traverses a different layer 15′ on the second side of metal core 10. Though not shown, second trace 12′ traverses a generally parallel but separate path as that of first trace 12.
FIG. 5 also shows the electric field distribution between the first and second trace 12, 12′. While traversing the same layer, there is unimpeded signal coupling between the signal carried on trace 12 and the signal carried on trace 12′, as shown by arrows 16 representing electric field lines. This is an optimal distribution, as there is little or no impedance mismatch since there is no conductive obstruction between first and second traces 12, 12′ to block the signal coupling. This holds true regardless of which layer is being traversed.
A problem arises, however, when the first and second traces 12, 12′ traverse through the metal core 10. A portion 18 of the metal core 10 (designated by dashed lines) separates the first and second vias 14, 14′, which impedes the signal coupling of the first and second traces 12, 12′. This results in several undesirable effects. First, as shown by electric field lines 20, there is impedance mismatch between the signals as they traverse first and second vias 14, 14′. Second, the common mode noise assumption no longer applies because the signals blocked by portion 18 of metal core 10 resulting in different degrees of noise couple to first and second traces 12, 12′ in differing amounts. These effects result in a significant degradation of the signal integrity, which impairs performance.
Accordingly, there is a need for apparatus and methods to employ differential signaling in conductive core substrates, which maintain matched impedance between the differential signal traces while traversing through the conductive core, while also maintaining the common mode noise assumption.